All right, okay: you're stuck. You're trying to use one of the tools on an image, and nothing is happening, and nothing you try makes any difference. Your fists are starting to clench, and your face is starting to feel warm. Are you going to have to kill the program, and lose all your work? This sucks!

Well, hold on a second. This happens pretty frequently, even to people who've used the GIMP for a long time, but generally the cause is not so hard to figure out (and fix) if you know what to look at. Lets be calm, and go through a checklist that will probably get you GIMPing happily again.